1. Field of the Invention
The present invention relates to a manufacturing method of a non-volatile semiconductor device and more particularly to a manufacturing method of a non-volatile semiconductor device characterized by a step of forming a tunnel insulating film.
2. Description of the Related Art
In a non-volatile semiconductor device such as a flash memory, an EPROM (Erasable Programmable Read-Only Memory) or the like, at the time of writing/erasing the information, electrons are implanted/discharged between the substrate and the floating gate through the tunnel insulating film. Because the tunnel insulating film is, in these instances, subjected to stress caused by high electric field, repetition of writing/erasing the information gives rise to a problem of advancing degradation of the tunnel insulating film and lowering hold characteristics of the information.
In recent years, there have been growing demands for higher-speed operations, together with those for an improvement of element reliability and an achievement of longer life. Writing/erasing at high speed, however, accelerates degradation of the tunnel insulating film. Moreover, while it is preferable to make the tunnel insulating film thin for attaining high speed, thinning the film leads to a lowering of reliability. In short, attainments of high speed and reliability run counter to each other, and, thus, the problem of degradation of the tunnel insulating film has become a matter of great concern in respect of improvements of not only element reliability and life span but also operational characteristics.
To overcome such problems, it has been so far proposed to use a silicon oxynitride film as the tunnel insulating film.
For example, in Japanese Patent Application Laid-open No. 193059/1995, it is disclosed that, on a silicon substrate, a silicon oxide film is formed by carrying out pyrogenic oxidation with oxygen gas, and thereafter heating at 1000xc2x0 C. is applied thereto in a closed pressure heating furnace while flowing through N2 and N2O at atmospheric pressure, and thereby a tunnel insulating film with a thickness of 7.5 nm or so is formed. It is further mentioned therein that NO or N02 can be employed as a nitriding gas, instead of N2O, and also the pressure of the nitriding gas can be lower than the atmospheric pressure. As for the temperature of heat treatment, it is described to set at 950xc2x0 C. to 1050xc2x0 C.
Further, Japanese Patent Application Laid-open No. 139437/1997, it is disclosed that a tunnel oxide film is formed as follows. Firstly, after a first silicon dioxide layer (with a thickness of 3.5 nm) is formed on a silicon substrate, annealing in argon atmosphere is applied thereto. Next, beneath this first silicon dioxide layer, a second silicon dioxide layer (with a thickness of 3 nm) is formed and then annealing in argon atmosphere is applied thereto. Thereafter, in N2O atmosphere, nitridation is carried out at 800xc2x0 C. to 1200xc2x0 C., whereby an oxynitride dielectric layer (a tunnel insulating film) with a thickness of 9.5 nm is obtained. It is also mentioned that NO can be employed instead of N2O.
In each of the above publications, as a method of forming a tunnel insulating film, there is described a method in which a silicon oxide film is formed on a silicon substrate and thereafter a heat treatment is carried out in nitriding atmosphere of N2O or the like. However, in forming a tunnel insulating film made of a silicon oxynitride film, to find out the right conditions for a heat treatment with a given heat treatment apparatus, under which prescribed element characteristics can be obtained, it is, hitherto, required to fabricate elements up to a certain stage once and then evaluate their element characteristics. In effect, it is necessary to repeat fabrication of elements and subsequent their evaluation till the very conditions for the heat treatment which can provide prescribed element characteristics can be found, and thus a considerable time and effort are needed to obtain appropriate conditions for a heat treatment. Further, even after conditions for a heat treatment are determined, because it cannot be known appropriately whether the formed tunnel insulating film can really demonstrate prescribed element characteristics or not, immediately after the tunnel insulating film is formed, there is always the possibility of proceeding formation of elements with poor reliability and, consequently, lowering the yield in production.
An object of the present invention is to provide a manufacturing method that can easily fabricate a non-volatile semiconductor device having a tunnel insulating film capable to demonstrate prescribed element characteristics.
The present invention relates to a method of manufacturing a non-volatile semiconductor device, having a floating gate overlying a tunnel insulating film set on a silicon substrate and a control gate overlying an inter-gate insulating film set on said floating gate; which comprises the steps of:
forming a silicon oxide film on the surface of the silicon substrate and subsequently applying a heat treatment thereto in atmosphere of a nitriding gas containing nitrogen oxide so as to form a nitridation region and thereby forming a tunnel insulating film; wherein
in said heat treatment performed in atmosphere of the nitriding gas, so that the maximum nitrogen atomic concentration in the tunnel insulating film that is to be formed may become equal to or greater than the maximum nitrogen atomic concentration(acceptable maximum nitrogen atomic concentration) capable to provide given acceptable holding characteristics, the pressure of said nitriding gas and the temperature of heat treatment are controlled on the basis of a pre-formed relationship equation between the thickness of the tunnel insulating film and the acceptable maximum nitrogen atomic concentration, for a prescribed thickness of the tunnel insulating film that is to be formed.
Further, the present invention relates to a method of manufacturing a non-volatile semiconductor device, having a floating gate overlying a tunnel insulating film set on a silicon substrate and a control gate overlying an inter-gate insulating film set on said floating gate; which comprises the steps of:
forming a silicon oxide film on the surface of the silicon substrate and subsequently applying heat treatment thereto in atmosphere of a nitriding gas containing nitrogen oxide so as to form a nitridation region and thereby forming a tunnel insulating film; wherein
in said heat treatment performed in atmosphere of the nitriding gas, the pressure of said nitriding gas and the temperature of heat treatment are controlled in such a way that there is formed, within said tunnel insulating film, a nitridation region having nitrogen atomic concentrations that satisfy the following Equation (1) for a prescribed thickness of the tunnel insulating film that is to be formed;
CNxe2x89xa7xe2x88x921.02T+13.4xe2x80x83xe2x80x83(1),
CN: the nitrogen atomic concentration (atomic %) in the tunnel insulating film
T: the thickness of the tunnel insulating film (nm).
The present invention makes it possible to fabricate easily a non-volatile semiconductor device having a tunnel insulating film capable to demonstrate prescribed element characteristics with high yield.